There are many known sensors using the excitation of surface plasmons, termed Surface Plasmon Resonance (SPR) Sensors, for detecting refractive index changes in a sample adjacent to the sensor surface. Such SPR sensors are used e.g. for quantifying concentrations of substances in chemical, biochemical, biological, biomedical or pharmaceutical research, in clinical or food diagnosis or in environmental measurements (e.g. detection of gas or wastewater), etc. Many SPR sensors can perform fast, parallel and massive inspections, which make these sensors also convenient for quantifying molecular interactions, in particular for studying the affinity and the real-time reaction kinetics between two or more interacting molecules.
SPR sensors rely on the well-known SPR phenomenon, which involves one or more surface-bond electromagnetic waves that propagate at an interface between a metallic material (typically gold or silver) and a dielectric material. Each surface-bond electromagnetic wave, which is due to a collective oscillation of free electrons at the metal-dielectric interface, propagates with its highest intensity parallel to this interface and decays exponentially away from this interface.
The most commonly used techniques for excitation of SPR exploit a prism in the Kretschmann configuration. In such case, the prism is covered with a noble metal layer supporting surface plasmons, and SPR is optically excited through the prism. Indeed, light can excite the resonance of surface plasmons at a metal-dielectric interface if an interface-parallel component of the incident light and a surface-bond electromagnetic wave of the SPR both have matching frequencies and matching wavelengths. In the resonance condition, the incident light is absorbed by the metal-dielectric interface so as to couple with the surface-bond electromagnetic wave. It is then possible to observe this absorption by detecting for example a reduction in the intensity of the light that is transmitted or reflected by the metal-dielectric interface. The coupling condition between light and surface plasmon waves being very sensitive to refractive index changes of the dielectric medium close to the metal-dielectric interface, SPR sensors take advantage of this sensitivity in the resonance coupling condition for detecting changes in the refractive index of a dielectric medium by measuring the decrease in intensity of light reflected from the metal-dielectric interface, while the latter is illuminated with an SPR exciting light beam.
SPR finds particular application in biosensor systems capable of detecting interactions between biomolecules or biochemical molecules, for example interactions between antigens and antibodies, enzymes and ground substances, endocrines and receptors, nucleic acids and nucleic acids, etc. In particular, many SPR biosensor systems have bio-receptors attached on their sensor surface so as to detect changes in the light-SPR coupling condition caused by refractive index changes at the sensor surface when biochemical molecules or biomolecules interact with (bind to) these bio-receptors. Such biosensor systems are suitable for measuring for example concentrations of biomolecules or biochemical molecules in solutions, etc.
Currently, there are a variety of laboratory equipment based on SPR sensing. US patent application No. 2009/021,727 describes bio-sensors based on the Kretschman configuration.
Another SPR biosensor system for detecting biochemical molecules is known from US∘2008/316,490 and employs a metal grating instead of a prism.
More recently, the discovery of localized surface plasmon resonance (L-SPR) phenomena and enhanced transmission through metallic subwavelength periodic structures, have shown great promise to significantly increase the size of the detection array, supporting high throughput applications. For L-SPR applications, the simplest and most versatile technology that has been explored in a broad range of technological areas is the so-called nanohole array sensing configuration. In its classical approach, the SPR sensor comprises a dielectric substrate covered with a layer of noble metal in which a periodic array of nanoholes is formed, i.e. holes having sub-wavelength dimensions.
Such L-SPR based sensors with nanohole arrays are e.g. described in WO2008/039212, WO2010/130045 and by Parsons, J. et al. in “Localized surface-plasmon resonances in periodic non-diffracting metallic nanoparticle and nanohole arrays” (PHYSICAL REVIEW B 79, 073412 (2009)).
Giudicatti, S. et al. in “Plasmonic resonances in nanostructured gold/polymer surfaces by colloidal lithography”, PHYSICA STATUS SOLIDI (A), vol. 297, April 2010 (April 2010), pages 935-942 describe a colloidal lithography procedure to prepare a LSPR supporting structure consisting of a gold film perforated by polymeric pillars arranged in a 2D hexagonal array.
The use of colloidal lithography in the preparation of SPR sensors is also disclosed in EP 2 264 438; and in “Bioadhesive nanoareas in antifouling matrix for highly efficient affinity sensors” by Mannelli et al., PROCEEDINGS OF THE SPIE—USA vol. 7035, 2008, pages 70350Y-1-70350Y-10.